year, yy, yyyy - The year part
quarter, qq, q - The quarter part
month, mm, m - The month part
dayofyear, dy, y - The day-of-year part
day, dd, d - The day part
week, wk, ww - The week part
weekday, dw, w - The weekday part
hour, hh - The hour part
minute, mi, n - The minute part
second, ss, s - The second part
millisecond, ms - The millisecond part

For example, DATEADD(year, 1, @birth_date) will increment 1 year to @birth_date.